The invention relates to semiconductor device fabrication and, more particularly, to techniques for CMP (Chemical Mechanical Polishing) and, more particularly, to ceria-based polish processes, particularly for CMP polishing of patterned oxides.
CMP is a key technology in the manufacture of high-density electronic circuits. The process uses advanced polishing techniques involving slurries—mixture of high-performance abrasives and chemicals. CMP technique is very important in the process for manufacturing a semiconductor device, for instance, shallow trench isolation (STI), planarization of interlayer dielectric, formation of embedded metal line, plug formation, formation of embedded capacitor, and the like.
A CMP process involves rotating a wafer against a polishing pad under pressure in the presence of a slurry. CMP is used to planarize silicon dioxide (“oxide”). For conventional oxide polishing, the slurry consists of a silica-based colloid suspended in a dilute alkaline solution (pH˜10–11). The polishing pad is typically a composite porous polyurethane-based material. The theory of oxide polishing is not well understood; however, it is generally accepted that the alkaline chemistry hydrolyzes the oxide surface and sub-surface thus weakening the SiO2 bond structure. The mechanical energy imparted to the colloid through pressure and rotation causes high features to erode at a faster rate than low features, thereby planarizing the surface over time.
The present invention is primarily directed to chemical mechanical polish (CMP) of oxides. In this process, wafers with silicon dioxide thin film are polished with the use of an abrasive mineral on a polishing pad. This art is practiced extensively in the glass polishing industry as well as in the semiconductor industry. The abrasive mineral is typically suspended in an aqueous medium that may also contain other chemicals to reduce the settling of the mineral and also to improve polish performance. The most commonly used abrasive minerals in the semiconductor industry for the purpose of planarizing silicon dioxide thin films are oxides of silicon and cerium. Cerium oxides are more popular for better planarizing performance, particularly for polishing patterned oxides. As used herein, patterned oxides include, but are not limited to, shallow trench isolation (STI), premetal dielectric (PMD) and interlevel dielectric ILD.
US Patent Application No. 2004/0040217 (“Takashina”) discloses a polishing composition comprising an aqueous medium and abrasive particles, wherein the abrasive particles comprise abrasive particles having a particle size of 2 to 200 nm in an amount of 50% by volume or more, the abrasive particles having a particle size of 2 to 200 nm comprising (i) 40 to 75% by volume of small size particles having a particle size of 2 nm or more and less than 58 nm; (ii) 0 to 50% by volume of intermediate size particles having a particle size of 58 nm or more and less than 75 nm; and (iii) 10 to 60% by volume of large size particles having a particle size of 75 nm or more and 200 nm or less; a polishing composition comprising an aqueous medium and abrasive particles, wherein the abrasive particles comprise abrasive particles (A) having an average particle size of 2 to 50 nm; and abrasive particles (B) having an average particle size of 52 to 200 nm, wherein a weight ratio of A to B (A/B) is from 0.5/1 to 4.5/1; a polishing process comprising subjecting a semiconductor substrate to planarization with the polishing composition; a method for planarization of a semiconductor substrate with the polishing composition; and a method for manufacturing a semiconductor device, comprising polishing a semiconductor substrate with the polishing composition. The polishing composition can be favorably used in polishing the substrate for a semiconductor device, and the method for manufacturing a semiconductor device can be favorably used for manufacturing a semiconductor device such as memory integrated circuits (IC), logic ICs and system large scale integrated circuits (LSI).
As noted in Takashina, one example of a polishing liquid for CMP includes a dispersion of abrasive particles in water. The abrasive particles include particles of fumed silica, alumina and the like. Among them, the fumed silica is widely used because of its low cost and high purity. However, there is a disadvantage in the fumed silica that scratches are likely to be generated on the surface because aggregated particles (secondary particles) are formed in the production process. On the other hand, since silica abrasive grains which are referred to as “colloidal silica” have relatively spherical surface shape, and are in a state of nearly monodisperse, aggregated particles are hardly likely to be formed, so that reduction in scratches can be expected and that the silica abrasive grains have begun to be used. However, there is a disadvantage in the silica abrasive grains that the polishing rate is generally low.
As noted in Takashina, particles of silicon dioxide, aluminum oxide and cerium oxide are preferable, and silicon dioxide is more preferable from the viewpoint of reducing scratches. Concrete examples include silicon dioxide particles such as colloidal silica particles, fumed silica particles and surface-modified silica particles; and cerium oxide particles such as those having an oxidation number of 3 or 4 and those having hexagonal, isometric or face-centered cubic crystal system; and the like.
As noted in Takashina, colloidal silica particles are more preferable. The colloidal silica particles have a relatively spherical shape, which can be stably dispersed in the state of primary particles, so that aggregated particles are hardly formed, whereby scratches on a surface to be polished can be reduced. The colloidal silica particles can be prepared by a sodium silicate method using an alkali metal silicate such as sodium silicate as a raw material, or an alkoxysilane method using tetraethoxysilane or the like as a raw material. These abrasive particles may be used alone or in admixture of two or more kinds.
As noted in Takashina, the polishing compositions can optionally contain various additives. The additives include a pH adjusting agent, a dispersion stabilizer, an oxidizing agent, a chelating agent, a preservative, and the like.
The pH adjusting agent includes basic substances such as an aqueous ammonia, potassium hydroxide, sodium hydroxide and water-soluble organic amines, and acidic substances including organic acids such as acetic acid, oxalic acid, succinic acid, glycolic acid, malic acid, citric acid and benzoic acid, and inorganic acids such as nitric acid, hydrochloric acid, sulfuric acid and phosphoric acid. Here, oxalic acid and succinic acid can be also used as a chelating agent.
The dispersion stabilizer includes surfactants such as anionic surfactants, cationic surfactants and nonionic surfactants, polymeric dispersants such as polyacrylic acids or salts thereof, acrylate copolymers and ethylene oxide/propylene oxide block copolymers (Pluronics), and the like.
The oxidizing agent includes peroxides, permanganic acid or salts thereof, chromic acid or salts thereof, nitric acid or salts thereof, peroxo acid or salts thereof, oxyacid or salts thereof, metal salts, sulfuric acid, and the like.
The chelating agent includes polycarboxylic acids such as oxalic acid, succinic acid, phthalic acid and trimellitic acid; hydroxycarboxylic acids such as glycolic acid, malic acid, citric acid and salicylic acid; polyaminocarboxylic acids such as nitrilotriacetic acid and ethylenediaminetetraacetic acid; phosphonic acids such as aminotri(methylenephosphonic acid) and 1-hydroxyethylidene-1,1-diphosphon-ic acid, and the like.
The preservative includes benzalkonium chloride, benzethonium chloride, 1,2-benzisothiazolin-3-one, and the like.
As noted in Takashina, it is preferable that the pH of the polishing compositions is appropriately determined depending upon the kinds of the objects to be polished and the required properties. For instance, it is preferable that the pH of the polishing composition is preferably from 2 to 12, from the viewpoints of the cleanability of the objects to be polished, the anti-corrosiveness of the working machine, and the safety of the operator. In addition, when the objects to be polished are used for polishing a semiconductor wafer, a semiconductor element, or the like, especially for polishing a silicon substrate, a poly-silicon substrate, a silicon oxide film, or the like, the pH is more preferably from 7 to 12, still more preferably from 8 to 12, especially preferably from 9 to 12, from the viewpoints of increasing the polishing rate and improving the surface qualities. The pH can be adjusted by adding the above-mentioned pH adjusting agent properly in a desired amount as occasion demands.
US Patent Application No. 2003/0211747 (“Hegde et al.”) discloses shallow trench isolation polishing using mixed abrasive slurries. Isolation of active areas, e.g., transistors, in integrated circuits and the like so that functioning of one active area does not interfere with neighboring ones, is provided by the shallow trench isolation technique followed by chemical-mechanical polishing with a mixed abrasive slurry consisting essentially of (a) relatively large, hard inorganic metal oxide particles having (b) relatively small, soft inorganic metal oxide particles adsorbed on the surface thereof so as to modify the effective charge of the slurry to provide more favorable selectivity of silicon dioxide to silicon nitride, the slurry having a pH below about 5.
As noted in Hegde, a mixed abrasive polishing slurry for the CMP process is provided consisting essentially of at least two inorganic metal oxide abrasive materials such as ceria (CeO2) and alumina (Al2O3) particles at a pH below 5, e.g. on the order of ˜4.0 or less in order to control the polish rate selectivity of oxide to nitride and to reduce surface defects.
The mixed abrasive slurries (“MAS”) will consist essentially of a mixture of (a) small and soft particles and (b) relatively large and hard particles as will be described in detail hereinafter. The two kinds of slurry particles are selected to provide opposite polarity such that the smaller particles are attracted to and thereby preferentially adsorbed on the surface of the larger particles.
The large, hard particles in the slurry, which provide the necessary mechanical abrasion function of the slurry, will, for example, possess a mean particle size of on the order of from about 80 to 250 nm while the small, soft particles adsorbed on the surface of the large particles may, for instance, be on the order of from about 10–40 nm. The ratio of large to small particles may, for instance, be on the order of from about 1:1 to about 1:5 by weight, preferably from 1:1 to 1:1.2 by weight.
As examples of useful large particles, mention may be made of alumina, iron oxide, chromia, ceria, titania, germania and zirconia; while examples of useful materials for the small particles include silica, zirconia and ceria.
The schematic polishing mechanism of this invention illustrated in FIG. 4 shows how the small abrasive particles cluster around the surface of the large abrasive particles. As postulated by Hegde, when the modified abrasive containing small, soft ceria particles adsorbed onto large, hard alumina particles interact with the silicon dioxide substrate, first the unique chemical interaction of ceria with the silicon dioxide substrate softens the surface film of SiO2 and then the hard alumina particles mechanically abrade the softened film. On the other hand, for Si3N4 to be removed at a faster rate, it has to be first oxidized to SiO2 and only then the synergistic interaction of large and small particles in the modified abrasive can come into play to enhance the RR. At pH values below 5, the oxidation of Si3N4 to SiO2 is very slow and hence the modified abrasive always interacts with hard Si3N4 surface film during polishing, leading to low Si3N4 RR and thus improving selectivity.
As noted in Hegde, since the ceria particles that interact with the polishing material are very soft, the resulting surface smoothness is excellent. This is the advantage of mixed abrasive slurry, where specific interactions between the abrasive and the polishing substrate can be appropriately tailored to control the removal rates of polishing materials to desirable values and simultaneously produce high post-polish surface quality (low surface roughness).
US Patent Application No. 2003/0092271 (“Jindal”) discloses shallow trench isolation polishing using mixed abrasive slurries. Isolation of active areas, e.g. transistors, in integrated circuits and the like so that functioning of one active area does not interfere with the neighboring ones, is provided by the shallow trench isolation technique followed by chemical-mechanical polishing with a mixed abrasive slurry consisting essentially of at least two inorganic metal oxide abrasive material particles at a pH below five, preferably on the order of 3.5 to 4.0, in order to control the polish rate selectivity of silicon dioxide to silicon nitride of the circuit and to reduce surface defects. The mixed abrasive polishing slurry for the CMP process consists essentially of at least two inorganic metal oxide abrasive materials such as ceria (CeO2) and alumina (Al2O3) particles at a pH below 5, e.g. on the order of about 4.0 or less in order to control the polish rate selectivity of oxide to nitride and to reduce surface defects.
As noted in Jindal, in the current generation devices, an improved isolation, greater packing density and superior dimensional control is achieved by the Shallow Trench Isolation method. Notably, Shallow Trench Isolation is formed by etching a trench through the silicon nitride and the silicon oxide layers into the silicon substrate to a predetermined depth. Silicon oxide is then deposited over the entire wafer and into the trench opening in the silicon nitride using a special technique known in the art as Chemical Vapor Deposition (“CVD”). Chemical-mechanical polishing is then applied to remove excess CVD silicon oxide and is stopped on the protective silicon nitride. The nitride is then etched out using strong, hot acids.
CMP must stop when the nitride layer is reached and this requires a very high oxide-to-nitride-selectivity-slurry for CMP. In the CMP process, as the name of the process infers, planarization is achieved through the contributions of both chemical reactions and mechanical abrasion. The chemical reactions take place between the slurry and the material being polished. Mechanical abrasion of the film is caused by the interaction between the pad, the abrasives and the film.
Accordingly, the three major components of a CMP process are the film, the pad and the slurry. Since the process is very well known in the art, including its essential components, it need not be discussed in much detail herein.
Of these three major components, it is stressed that the use of highly selective slurries which yield minimal defects in the shallow trench isolation procedure is by far the most critical for providing a commercial product. Accordingly, it is stressed that providing highly selective slurries which yield minimal defects after chemical-mechanical polishing of the shallow trench isolation is essential for a vitally important and commercial shallow trench isolation system.
US Patent Application No. 2003/0047710 (“Babu et al.”) discloses chemical-mechanical polishing. An abrasive slurry for chemical-mechanical polishing, e.g. to planarize metal and silicon wafers employed in the fabrication of microelectric devices and the like, the slurry consisting essentially only of a mixture of at least two inorganic metal oxides to provide superior performance in properties such as improved oxide and metal polish rates, controlled polish rate selectivity, low surface defectivity and enhanced slurry stability over that obtainable with a single inorganic metal oxide abrasive material.
As noted in Babu, the abrasives in the slurry play the very important role of transferring mechanical energy to the surface being polished. Illustrative abrasives for this purpose include silica (silicon dioxide, SiO2) and alumina (aluminum oxide, Al2O3). Ceria (cerium dioxide, CeO2) is the most popular abrasive for the polishing of the glass and (recently) oxide films for STL.
The abrasive-liquid interactions, through chemical and physical actions, play a very important role in determining the optimum abrasive type, size, shape and concentration. The abrasives, however, can also have a chemical effect as in the case of glass polishing with ceria abrasives where the ceria forms a poorly understood chemical bond with the glass surface.
As noted in Babu, CMP is per se old and has, for example, been used in glass polishing and silicon wafer polishing prior to integrated circuit fabrication for quite some time. Silicon dioxide employed in integrated circuit manufacturing is a form of silicate glass, so the two materials have similar mechanical and chemical properties. Silicon dioxide polishing is typically performed near pH 10 because of enhanced dissolution and chemical interaction in that pH regime. The dissolution rate is enhanced by the compressive stress that occurs during particle abrasion. Ceria and zirconia have been found to possess a special “chemical tooth” property that increases the silicon oxide removal by many orders of magnitude.
As noted in Babu, conventionally, silica particles alone have been used as the abrasives for silicon oxide and nitride polishing. Ceria-based slurries, which have high removal rates of oxide and high polish rate selectivity of oxide to nitride often cause slurry-induced scratches on the oxide surface. These scratches are detrimental to proper functioning of the integrated circuit devices. Deep scratches should be eliminated because they may attack the silicon substrate and negate oxide integrity.